Hearing-impaired (HI) listeners are severely disadvantaged in noisy situations. Fewer than 30% of hearing-aid users are satisfied with the performance of their devices in noise, even though satisfaction levels are considerably higher for less adverse conditions (Kochkin, 2000). These difficulties are compounded in fluctuating backgrounds. While normal-hearing (NH) individuals are able to take advantage of momentary dips in the level of a masker to receive a significant (5-10 dB) fluctuating-masker benefit (FMB) to speech intelligibility relative to stationary noise, HI listeners seem unable to do so (Festen and Plomp, 1990). The proposed research aims to elucidate the mechanisms responsible for the reduced FMB in HI listeners, setting the stage for the development of signal processing algorithms to target these specific mechanisms. Attempting to explain the limited FMB in HI listeners, past studies have focused on reduced audibility, reduced spectral or temporal resolution, or limited cues for target-source separation. This proposal explores the hypothesis that differences in the signal-to-noise ratio (SNR) at which HI and NH listeners are tested contribute to FMB differences, and for some fluctuating maskers may account for most of the reduction in FMB for HI listeners. An SNR-dependent FMB is predicted by an existing model of speech intelligibility (Rhebergen et al., 2006), if the effective speech dynamic range is assumed to be narrower for modulated maskers than previously estimated for stationary noise. Experiment 1 will directly measure this effective dynamic range to refine the model and improve the accuracy of FMB predictions across SNRs. Preliminary results indicate that after SNR differences are controlled, HI and simulated HI (HISIM) listeners show a similar FMB to NH listeners for certain fluctuating maskers. Experiments 2 and 3 will differentiate fluctuating-maskers types based on the extent to which the FMB is still reduced after SNR and audibility are equalized between listener groups. This proposal has the potential to substantially impact research efforts to improve speech intelligibility in noise. For fluctuating maskers where SNR effects do not account for the full magnitude of FMB differences, the methods developed here could control SNR differences to more directly pursue impairment-related distortions responsible for limiting FMB. For fluctuating maskers where HI listeners are shown to benefit from masker fluctuations as much as NH listeners after SNR differences are controlled, future work would seek to (a) improve target speech audibility, e.g. via fast compression, which could selectively amplify a low-level target in a fluctuating background and (b) identify factors limiting intelligibility in noise, generally, with the idea that the findings should also extend to fluctuating maskers. Furthermore, the refined speech intelligibility model has the potential to improve the clinical management of HI listeners via (a) its use in the development of signal processing algorithms to improve speech intelligibility and (b) its clinical application in identifying individuals likely to suffer distortions beyond audibility that limit speech intelligibility in fluctuating backgrounds. PUBLIC HEALTH RELEVANCE: Hearing-impaired listeners experience the most difficulty when trying to listen in noisy environments, particularly those environments with masking sounds that fluctuate in intensity, like interfering speech. This proposal seeks to understand and model the underlying causes of these particular difficulties. The knowledge gained and the computational model developed over the course of the project could significantly impact the direction of research and rehabilitation efforts aimed at alleviating the problems experienced by impaired listeners in noisy environments.